Current ultra large scale integrated circuits have a tendency that the packing density thereof is increased, and various fine processing techniques are being researched and developed. The existing design rule is a rule in the order of sub-half-micron. One technique developed to satisfy such a severe desired for the shrinkage of the size is CMP (chemical mechanical polishing) technique. In the process for producing semiconductor devices, this technique makes it possible to flatten completely layers to be exposed to light, decrease a burden on exposure technique, and make the yield stable; therefore, the technique becomes essential at the time of flattening an interlayer insulating film or a BPSG film, performing shallow trench isolation, and attaining others.
Hitherto, a fumed silica based polishing slurry has been generally investigated as a CMP polishing slurry for flattening an inorganic insulator film, such as a silicon oxide insulator film, which is formed by plasma CVD (chemical vapor deposition), low-pressure CVD or some other method. The fumed silica based polishing slurry is produced by growing silica-grains in a manner of the thermal decomposition of silicon tetrachloride or in some other manner, and then adjusting the pH thereof. However, such a polishing slurry does not have a sufficient polishing rate for polishing inorganic insulator films, and thus has a technical problem of a low polishing rate against the practical use thereof.
About conventional techniques for flattening an interlayer insulating film, the polishing rate has dependency on the pattern of a film to be polished on a substrate. Thus, in accordance with the difference in the density of the pattern or the difference in the size thereof, the rate of polishing convex regions therein is largely varied and further the polishing of concave regions therein also advances unfavorably. Consequently, there remains a technique problem that the whole of a surface of a wafer cannot be flattened at a high level.
In the CMP technique for flattening an interlayer insulating film, it is necessary to end the polishing in the middle of the interlayer insulating film, and a process-controlling method of controlling the polish amount dependently on polishing time is generally performed. However, the polishing rate is remarkably changed by a change in the shape of steps in a pattern, the condition of a polishing cloth, and others. Accordingly, the technique has a problem that the process thereof is not easily controlled.
In the generation of a 0.5-μm or more design rule, LOCOS (local oxidation of silicon) was used to isolate elements in integrated circuits. When the dimension for working is made finer thereafter, a technique for making the isolation width of elements narrow is required. Thus, shallow trench isolation is being used. In the shallow trench isolation, CMP is used to remove an excess of a silicon oxide film formed on a substrate, and a stopper film, about which the rate that this film is polished is slow, is formed beneath the silicon oxide film in order to stop the polishing. In the stopper film, silicon nitride or the like is used, and it is desired that the ratio between the rate that the silicon nitride film is polished and the rate that the stopper film is polished is large.
Meanwhile, a cerium oxide polishing slurry is used as a glass surface polishing slurry for photo-masks, lenses and others. Cerium oxide particles have a lower hardness than silica particles or alumina particles. Accordingly, polished surfaces are not easily scratched. For this reason, the polishing slurry is useful for mirror-finish polishing. Attention is paid to this matter to suggest a cerium oxide polishing slurry (Japanese Patent Application Laid-Open No. 2002-353175). However, this cerium oxide polishing slurry has a problem that flattening performance, which has been severely desired in recent years, is not always sufficient.
In light of the above-mentioned problems, the present invention provides a polishing slurry and polishing method which are capable of polishing a silicon oxide film effectively and rapidly and which also make the process-control of the polishing easy in CPM technique for flattening an interlayer insulating film, a BPSG film and an insulator film for shallow trench isolation.